Firstly, sorry for the tl;dr post.
>>1
That depends. Do you already know how to program?
If you are already reasonably experienced in at least one other language (though, the more languages you know, and by know, i mean that you understand and can use most of the languages features and can write large programs in said language, the better) then, yes, you should now learn C++.
If you do not already know another language (ie, you are asking if you should learn C++ as your first language), then I would say NO, do not learn C++ (not YET), learn another language first and practice writing games in that language first (you will not NEED C++ until you want to get a job anyway, nothing you do on your own will require C++). Though /prog/ will hate me for suggesting it, I'd say start with Python (pygame can be used to create a window, input, 2D bitmapped graphics and pyopengl can be used for 3D graphics).
This means you can learn programming concepts (and game programming concepts, which are somewhat different from "normal" programming) wihtout worrying about low level crap like memory management. Anything you learn in Python can be carried over to C++ later and you can even use Python as a scripting language from within a C++ program when the time comes (or write extensions for Python in C++, if you so prefer).
If you already know another language, but not very well, then you may as well stick with that language instead of learning a new one. Do what I suggested in the previous section, only with whatever language you have been learning, instead of Python.
>>3
If you have to ask, then you are NOT ready to write an MMO of any kind PERIOD. In fact, if you didn't have to ask, you would know that even with a ton of cash and an experienced team, the odds are against you. It takes experience (In the order of many years), money (not entirely true, Eternal Lands managed with minimal financial resources, but I'd say thats the exception, not the rule), a team of programmers/artists/designers/musicians/testers and a lot of time.
>>4
The performance argument breaks down regardless because no matter what a beginner writes, they are not going to benefit from C++. Either their games will simply not benefit from the performance (because they don't do enough to make it noticeable) or the beginner programmer will simply write inefficient C++ code and negate the perceived higher performance of the language ebcause it is much easier to write low performance C++ code than high performance since an inefficient algorithm will run slow regardless of the language and it takes experience to learn to improve algorithms.